Clathrate hydrates are crystalline compounds which occur when water forms a cage-like structure around guest molecules. Clathrate hydrates, especially in the hydrocarbon industry, are often referred to as gas hydrates, or simply as hydrates. Gas hydrates of interest to the hydrocarbon industry, particularly with respect to producing, transporting, and processing of natural gas and petroleum fluids, are composed of water and the following eight guest molecules: methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfide. Other guest molecules capable of forming clathrate hydrates, although not normally of significant interest to the hydrocarbon industry, include nitrous oxide, acetylene, vinyl chloride, methyl bromide, ethyl bromide, cyclopropane, methyl mercaptan, sulfur dioxide, argon, krypton, oxygen, xenon, trimethylene oxide, and others. Clathrate hydrate formation is a possibility any place water exists in the vicinity of such molecules, both naturally and artificially, at temperatures above 32.degree. F. and below 32.degree. F. when the pressure is elevated.
It is primarily due to their crystalline, insoluble, non-flowing nature that hydrates have been of interest to industry. They have been considered a nuisance, because they block transmission lines, plug Blow Out Preventors, jeopardize the foundations of deepwater platforms and pipelines, collapse tubing and casing, and foul process heat exchangers and expanders. Common examples of preventive measures are found in the regulation of pipeline water content, unusual drilling mud compositions, and large quantities of methanol injection into pipelines.
Hydrates normally form in one of two small, repeating crystal structures. Structure I (sI), a body-centered cubic structure, forms with natural gases containing molecules smaller than propane. Structure II (sII), a diamond lattice within a cubic framework, forms when natural gases or oils contain molecules larger than ethane but smaller than pentane; this structure represents hydrates which commonly occur in hydrocarbon production and processing conditions. Also, at least one other repeating crystal structure is known to exist, and additional structures theoretically could exist.
The structures of both sI and sII are given with reference to a water molecule skeleton, in which guest molecules are encaged, composed of a basic "building block" cavity which has twelve pentagonal faces given the abbreviation 5.sup.12. By linking the vertices of 5.sup.12 cavities one obtains sI, while linking the faces of 5.sup.12 cavities results in sII. The regions between the linked 5.sup.12 cavities in repeating crystal structures are larger cavities which contain twelve pentagonal faces and either two or four hexagonal faces: 5.sup.12 6.sup.2 for sI and 5.sup.12 6.sup.4 for sII. The water molecules around a cavity are held in place by hydrogen bonds, which attach water molecules to each other to form the cavity. Inside each cavity resides a maximum of one guest molecule. Cavities other than the 5.sup.12,5.sup.12 6.sup.2, and 5.sup.12 6.sup.4 cavities, just described, could exist in repeating crystal structures other than sI and sII. Any cavity, however, should have exactly twelve pentagonal faces. Additional information concerning clathrate hydrates, and particularly gas hydrates, can be found in Sloan, Clathrate Hydrates of Natural Gases, M. Dekker, N.Y., 1990, the contents of which are incorporated herein in its entirety.
There are four common means of inhibiting formation of or dissociating hydrates, namely: 1) removing one of the components, either the guest molecule or water, 2) heating the system beyond the hydrate formation temperature at a given pressure, 3) decreasing the system pressure below hydrate stability at a given temperature, and 4) injecting an inhibitor such as methanol or glycol to alter hydrate stability conditions so that higher pressures and lower temperatures are required for hydrate stability. The above four common techniques are termed thermodynamic inhibition, because they remove the system from thermodynamic stability, by changes in composition, temperature, or pressure. While the system is kept outside thermodynamic stability conditions, hydrates can never form.
Recently, new chemical treatments have been proposed for controlling clathrate hydrate problems. For Example, in European Patent Office Publication No. 0457375A1, published Nov. 21, 1991, use of alkyl aryl sulfonic acids is proposed for preventing or retarding the formation of hydrates or for reducing the tendency of hydrates to agglomerate. Additionally, International Publication No. WO 93/25798, published Dec. 23, 1993, proposes the use of polymers of N-vinyl-2-pyrrolidone for inhibiting the growth and/or agglomeration of gas hydrate crystals.
Also, in U.S. Pat. No. 4,915,176 by Sugier et al., issued Apr. 10, 1990, a method is proposed for using amphophilic compounds, having a hydrophilic part and a lipophilic part, which are mixed with a fluid to be transported to lower the gas hydrate formation temperature and/or to modify the mechanism of formation of such hydrates. It is reported that such compounds disperse the gas hydrates in the fluid and then prevent their agglomeration. However it is believed that the process would not be effective in a fluid system containing a continuous aqueous liquid phase, that is, for example, an aqueous liquid phase that is not dispersed throughout a continuous organic liquid phase such as in a water-in-oil type emulsion. Therefore, that process would not be effective, for example, in a fluid system containing both a gaseous and an aqueous liquid phase, but containing no organic liquid phase, or a fluid system comprising significantly more aqueous liquid phase than organic liquid phase.
Many of these recently proposed chemical treatments are expensive due to the high cost of the chemicals and/or the relatively low effectiveness of the chemicals. International Publication No. WO 94/12761, published on Jun. 9, 1994, describes the use of certain polymers having lactam rings. These additives represent an improvement in both performance and cost, but are still relatively expensive. Also, many of these recently proposed methods are not adequate to control problems with hydrates at the very high pressures and low temperatures that are encountered in many situations, such as during subsea pipeline transportation. For example, subsea pipelines may be operated at pressures as high as 2000 psi or more at temperatures of 40.degree. F. or less, but the effectiveness of many hydrate control methods are limited to much lower pressures at low temperatures.
There is a need for new, more effective treating chemicals and less expensive treating methods to address clathrate hydrate problems in fluid systems, particularly those encountered in producing, transporting, and processing petroleum and natural gas fluids.